>Hi
>I have tried them
>
>

Modification in Machine.Config, Web.Config or Global.asax files
>Application's bin directory or its contents modified 
>Antivirus is running on the  .config /.aspx files
>The number of re-compilations (aspx, ascx or asax) exceeds the limit specified by the <compilation numRecompilesBeforeAppRestart=/> setting in machine.config or web.config  (by default this is set to 15) 
>The physical path of the virtual directory is modified 
>The CAS policy is modified
>Sub-Directories of Application are deleted or renamed

>
>nothing changed
>
>

App domain restarts

>
>will check on it
>
>

"System.Web.HttpException: Request timed out."

>
>not getting this error
>
>

<sessionState cookieless="false" timeout="330"/>

>
>session expired randomly not happen so often, but still happen
>In fact, we have set this programmatically at Session_Start event
>
>

memory recycling

>
>Disabled
>
>

Worker process recycled in min

>
>Default, 17xx mins
>
>

Antivirus

>
>Not installed
>
>This issue happen even to a laptop at support site, but not in our testing environment
>
>Any ideas?
>
>Thank you

As before: Log any application recycling that occurs. If it *is* recycling unexpectedly then you will know what is causing it. If it is not then you will know that the problem lies elsewhere

http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/87892589-4eda-4003-b4ac-3879eac4bf48.mspx?mfr=true